Electronics chopper circuits and switching between traction mode and braking mode by electronic components are known. Known circuits use choppers capable of operating both in step-up (booster) and in step-down modes, and therefore require complicated electronic or electromechanical circuits to switch from step-down to booster mode and vice-versa. In the prior art, heavy and bulky electromechanical circuits are used to switch from the traction mode to the braking mode and vice versa. Likewise, the operating direction of the machine is reversed in the prior art by electromechanical apparatus.
In the present invention which remedies these drawbacks, the electronic chopper operates as both a step-down chopper and a booster chopper and feeds a separately excited DC machine. It uses entirely electronic means to enable the DC machine to operate either in drive mode or in regenerator mode. Preferred embodiments of the invention also provide static switching for reversing the operating direction (forward or reverse) of the DC machine, and for varying the effective rheostatic braking resistance.
The present invention provides an electronic circuit for a separately excited DC machine that includes an armature and a field winding arranged for separate excitation, the electronic control circuit comprising:
positive and negative input terminals for connection to a source of smoothed DC;
first and second armature output terminals for connection to the armature of the DC machine;
a step-up/step-down chopper circuit connected to said input terminals and to said armature output terminals;
a logic control circuit connected to the chopper circuit;
a rheostatic braking circuit connected between the armature output terminals; and
a reverser circuit having first and second field winding output terminals for connection to the field winding of the DC machine, input terminals for connection to a source of electrical power and switching means for connecting DC of a selectable polarity to the field winding output terminals;
the improvement wherein the chopper circuit comprises:
a capacitor bridge connected across the positive and negative input terminals, the capacitor bridge comprising a first capacitor and a second capacitor connected in series to provide a capacitor bridge mid-point terminal;
a traction/braking bridge connected across the positive and negative input terminals, the traction/braking bridge comprising a traction arm and a braking arm connected in series to provide a traction/braking bridge mid-point terminal;
each arm of the traction/braking bridge comprising a respective thyristor connected in series with a respective protective inductor and in parallel with a respective diode, the traction and braking thyrsistors both being connected to conduct electricity selectively in the direction from the positive input terminal towards the negative input terminal, and the traction and braking diodes being connected with the opposite direction of conductivity; and
a turn-off circuit connected in series between the mid-point terminals of said bridges, the turn-off circuit comprising a turn-off inductor connected in series with a bi-directional, static, turn-off switch; one of the armature output terminals being connected to same one of the positive and negative input terminals as is connected to the braking arm of the traction/braking bridge, and the other armature output terminal being connected to the mid-point of the traction/braking bridge, with an armature current smoothing inductor being connected in series in the armature circuit; and
the logic control circuit being operatively arranged within the circuit so as to cyclically switch on the thyristor in a selected one of the arms of the traction/braking bridge, and then to switch on the turn-off switch in order to cause said selected thyristor to be turned-off, the on-duty ratio of the traction thyristor determining the tractive effect and the off-duty ratio of the braking thyristor determining the regenerative braking effect.
Preferably said reverser circuit comprises an AC electric source which feeds a primary winding of a transformmer having a centre-tapped secondary winding with its ends connected to respective pairs of thyristors connected head-to-tail, each thyristor pair being connected to one of said field winding output terminals with the other field winding output terminal being connected to the mid point of said secondary winding, said logic control circuit being connected to select one thyristor from each pair to determine whether the DC machine is to operate in the forward or reverse direction.
Preferably said rheostatic braking circuit comprises a braking thyristor connected in series with a braking resistance, said logic circuit causing the braking thyristor to conduct with a variable duty ratio during rheostatic braking.
Naturally the combination of both the static switched reverser circuit and the static, switched rheostatic braking circuit is advantageous in practice.
In any of the above arrangements the bi-directional, static, turn-off switch may advantageously comprise a full-wave rectifier bridge with its AC terminals connected in series in the turn-off circuit and its DC terminals connected to be selectively shorted together by a turn-off thyristor.